Add a whats-new entry

docs/source/whatsnew/pr/sunken-brackets.rst

@@ -0,0 +1,7 b''
+Don't start a multiline cell with sunken parenthesis
+----------------------------------------------------
+
+From now on IPython will not ask for the next line of input when given a single
+line with more closing than opening brackets. For example, this means that if
+you (mis)type ']]' instead of '[]', a ``SyntaxError`` will show up, instead of
+the ``...:`` prompt continuation.